In the conventional operation of a gas decomposition reactor such as a fluidized bed reactor that produces polysilicon, input gas containing hydrogen and a silane (any of SiH4, and its halogen derivatives may be used) suspends a column of seed particles in a heated reactor. The composition and flow velocity of the gas are critical parameters and operate to keep the particles suspended without fusing to each other. Currently, the conversion efficiency of the process is determined by large scale mass balance.
The gas is reactive, which may cause decomposition to occur before the gas reaches a mass flow controller, or may cause decomposition inside the mass flow controller itself. Mass flow controllers operate on Poiseuille's principle with measurements proportional to the 4th power of the diameter of a capillary. Therefore, mass flow controllers are very sensitive to deposition or corrosion inside the capillary. Corrosive gases can damage the sensor and bring it out of calibration. This is a frequent occurrence that is well known in the semiconductor industry.
Further, calibration of mass flow controllers, typically done with N2 or dry air reference whose viscosity is known, does not necessarily translate to an accurate calibration for gases whose viscosities are not well known. The only experimental data found for the viscosity of silane, in open scientific literature, is two data points measured in 1921. There is no other data to compare to see if there was an error of several percent. Therefore, flow rates cannot be assured of being translated into precise compositions. Accordingly, there exists a need for a practical gas measurement system to independently check the incoming gas composition that is sensitive to the molar gas density, can simultaneously detect hydrogen and silane, and whose relative sensitivities to the gases is extremely stable in the ratiometric sense and thereby insensitive to drift in absolute calibration.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.